Structural evolution of the intergrowth bismuth-layered Bi7Ti4NbO21

A series of intergrowth bismuth-layered ferroelectric Bi7Ti4NbO21 materials are reactive-sintered at 1050 to 1150 °C from Bi3TiNbO9 and Bi4Ti3O12 parent phases to infer their structural characters and microstructure relations. Various types of stacking faults are revealed in the intergrowth structure with extra Bi3TiNbO9 or Bi4Ti3O12 layer(s) by high-resolution transmission electron microscopy; some faults with even spacing form locally new intergrowths of Bi10Ti5Nb2O30 and Bi11Ti7NbO33. Co-growth of Bi7Ti4NbO21 epitaxially grown onto the remaining Bi4Ti3O12 grains is found in the low temperature sintered samples, while the Bi4Ti3O12 co-growth onto the intergrowth grains is also found in the high temperature samples. Both co-growths are created from intergranular melts during a solution-precipitation process, which is consistent with the anisotropic growth of the intergrowth structure and the presence of a Bi-rich intergranular phase. The populations of different stacking faults are found to decrease with the increase of their thickness and also with the increase of sintering temperature, indicating that they are remnants survived from dissolution to imbed via precipitation into the intergrowth structure, which should be created from the smaller but much abundant one-layered remnants of the parent phases. This leads to a new model of structural reorganization by such one-layered units to form the intergrowth structure in this solution-precipitation process. Such incomplete dissolution is initiated by the preferential melting of interleaved [Bi2O2]2+ sheets to enable the exfoliation of perovskite layers to re-order into the intergrowth structure. This reorganization model re-defines the reactive sintering as an evolution process of Bismuth-layered structures.